How to use sub-menus on the content page.

I have a page with three lines of PAGE TEXT. I have now created three sub-menus under this page each having a few lines of text. Currently when I am clicking on the sub-menu my PAGE TEXT is disappearing. Is it possible to keep the main PAGE TEXT fixed, while showing different sub-menu items. Can someone please share the code and tell me where to place it. Thank you.

Thank you so much guys for reading this post. I have found a very simple solution to this. Add the following article code from Page.ss to SideBar.ss in the top area and place the aside unit size below the article code. Thats it! It works beautifully. Hope this helps!